GB2312991A - Semicondcutor laser - Google Patents

Abstract

The device comprises a first lower cladding layer 112 lattice matched to a GaAs substrate 101, a second lower AlGaAs cladding layer 113 having a larger band gap energy disposed on the first cladding layer, an active layer 104 having a band gap energy less than the lower cladding layer, a first upper cladding layer 114 disposed on the active layer and having a larger band gap energy than the second upper cladding layer 117 and the active layer, the second cladding layer being lattice matched to the GaAs substrate and having a band gap energy greater than the active layer. The first lower cladding and the second upper cladding layers comprise InGaP. In this structure the stress within the crystalline structure of the device is reduced which improves the reliability of the device.

Description

SEMICONDUCTOR LASER DEVICE AND METHOD FOR FABRICATING SEMICONDUCTOR LASER DEVICE FIELD OF TRF INVENTION The present invention relates to a 0.78 urn band buried ridge type semiconductor laser device and a method for fabricating the semiconductor laser device.

BACKGROUND OF THE ISENTTON Figure 7 is a cross-sectional view illustrating a 0.78 pm band buried ridge type semiconductor laser device according to a prior art. In the figure, reference numeral 1 designates an n type GaAs substrate having opposed front and rear surfaces. An n type GaAs buffer layer 2 having a thickness of 1 um is disposed on the front surface of the substrate 1. An n type AlxGal~xAs (Al composition x = 0.48) lower cladding layer 3 having a thickness of 1.5 urn is disposed on the buffer layer 2. An active layer 4 is disposed on the lower cladding layer 3. The active layer 4 has a quantum well structure in which AlxGal~xAs (x = 0.1) well layers and AlxGal~xAs (x = 0.35) barrier layers are alternatingly laminated. A p type AlxGal~xAs (x = 0.48) first upper cladding layer 5 having a thickness of 0.3 urn is disposed on the active layer 4. A p type AlxGal~xAs (x = 0.7) etch stopping layer 6 having a thickness of 0.02 urn is disposed on the first upper cladding layer 5. A p type AlxGal~xAs (x = 0.48) second upper cladding layer 7 having a thickness of 1.2 pm is disposed on a central portion of the etch stopping layer 6. A p type GaAs cap layer 8 having a thickness of 1 pm is disposed on the second upper cladding layer 7. The second upper cladding layer 7 and the cap layer 8 form a stripe-shaped ridge structure. N type GaAs current blocking layers 9 having a thickness of 1 urn are disposed on the etch stopping layer 6 at both sides of the ridge structure. P type GaAs layers 10 having a thickness of 1.2 pm are disposed on the current blocking layers 9. A p type GaAs contact layer 11 having a thickness of 2 pm is disposed on the cap layer 8 and on the p type GaAs layers 10. An n side electrode 20 comprising AuGe/Ni/Au is disposed on the rear surface of the substrate 1. A p side electrode 21 comprising Cr/Au is disposed on the contact layer 11.

Figures 8(a)-8(d) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser device shown in figure 7. In the figures, reference numeral 18 designates an SiO2 film.

Initially, there are successively grown on the n type GaAs substrate 1 by MOCVD (Metal Organic Chemical Vapor Deposition), the n type GaAs buffer layer 2, the n type AlGaAs lower cladding layer 3, the quantum well active layer 4, the p type AlGaAs first upper cladding layer 5, the p type AlGaAs etch stopping layer 6, the p type AlGaAs second upper cladding layer 7, and the p type GaAs cap layer 8 (figure 8(a)).

Thereafter, an SiO2 film is deposited on the p type GaAs cap layer 8 by sputtering and patterned to form a mask 18. Using the mask 18, the p type GaAs cap layer 8 is etched with a mixture of NH40H and H202 and, subsequently, the second upper cladding layer 7 is wet-etched, thereby producing a ridge structure comprising the cap layer 8 and the second upper cladding layer 7 (figure 8(b)). The wetetching is stopped at the etch stopping layer 6.

Next, the n type GaAs current blocking layer 9 and the p type GaAs layer 10 are successively grown on the etch stopping layer 6 contacting the both sides of the ridge structure by MOCVD (figure 8(c)).

After removal of the SiO2 mask 18 with HF, the p type GaAs contact layer 11 is grown on the p type GaAs cap layer 8 and on the p type GaAs layer 10 by MOCVD. To complete the laser structure, the n side electrode 20 is formed on the rear surface of the GaAs substrate 1 and the p side electrode 21 is formed on the contact layer 11 (figure 8(d)).

A description is given of the operation of the 0.78 urn band AlGaAs semiconductor laser device. When a forward bias voltage is applied across the p side electrode 21 and the n side electrode 20, holes are injected into the active layer 4 from the p side electrode 21 through the p type layers 11, 8, 7, 6, and 5, and electrons are injected into the active layer 4 from the n side electrode 20 through the n type layers 1, 2, and 3. The injected charge carriers, i.e., electrons and holes, are confined within the active layer 4 and recombine to produce laser light, and this laser light is also confined within the active layer 4. The confinement of the charge carriers and the laser light within the active layer 4 is attributed to the fact that the energy band gap of the active layer 4 is smaller than the energy band gaps of the lower cladding layer 3 and the upper cladding layers 5 and 7 and, therefore, the refractive index of the active layer 4 is larger than the refractive indices of those cladding layers 3, 5, and 7. When the current flowing between the electrodes 21 and 20 exceeds a threshold current level, the laser device oscillates and outputs laser light continuously. The wavelength of the generated laser light is proportional to the inverse of the energy band gap of the active layer 4. Therefore, in the prior art semiconductor laser device, the active layer 4 is made of AlGaAs whose energy band gap can be changed by changing the Al composition, and the Al composition is selected so that the laser produces a laser light having a wavelength of 0.78 urn.

More specifically, the active layer 4 comprises alternating AlxGal~xAs (x = 0.1) well layers and AlxGal~xAs (x = 0.35) barrier layers.

The above-described prior art semiconductor laser device has the following drawbacks.

Since the lattice constant of the AlGaAs layers for the lower cladding layer 3 and the upper cladding layers 5 and 7 is larger than the lattice constant of the GaAs substrate 1 and the GaAs contact layer 11, the laser device has a stress within the crystalline structure. The amount of the stress depends on the compositions and the thicknesses of the cladding layers 3, 5, and 7. In the semiconductor laser device having an oscillation wavelength of 0.78 urn, in order to prevent overflow of charge carriers and improve the temperature characteristics of the laser, it is necessary to make the energy band gaps of the cladding layers 3, 5, and 7 larger than an energy band gap that is equivalent to 615 nm.

When the cladding layers comprise AlGaAs, the Al composition must be larger than 0.47. In this case, the difference between the lattice constant of the AlGaAs cladding layers and the lattice constant of the GaAs substrate 1 and the GaAs contact layer 11 exceeds 0.06 %.

Furthermore, the lower AlGaAs cladding layer 3 and the upper AlGaAs cladding layers 5 and 7 are sandwiched by the GaAs layers 1, 2, 8, and 11. In this case, if the thickness of the lower cladding layer 3 or the total thickness of the upper cladding layers 5 and 7 is thinner than 1.5 Fm, laser light exudes and reaches the GaAs layers. Since the energy band gap of GaAs is smaller than the energy band gap that is equivalent to the oscillation wavelength of the laser, the laser light is absorbed in the GaAs layers, resulting in a high oscillation threshold. Therefore, in the prior art semiconductor laser device, the thicknesses of the AlGaAs layers 3, 5, and 7 that have the lattice constant about 0.06 % different from the lattice constant of the GaAs substrate 1 and the GaAs contact layer 11 must be 3 urn or more in total at the upper and lower sides of the active layer 4, whereby a considerable stress is applied to the active layer 4. This stress causes crystalline defects, i.e., dislocations, in the active layer 4, whereby the laser characteristics are significantly degraded. That is, the injected charge carriers in the dislocations recombine, and the recombination of charge carriers increases the operating current and breeds the dislocations that cause further increase in the operating current. As a result, the reliability of the semiconductor laser device is degraded.

Furthermore, the p type AlxGal~xAs (x = 0.7) etch stopping layer 6 is employed to improve the controllability of the wet etching when the ridge structure is produced, and actually the controllability is significantly improved by the etch stopping layer 6. However, since the etch stopping layer 6 has such high Al composition, the surface of the etch stopping layer 6 is easily oxidized and an oxide film is formed at the surface. The oxide film adversely affects the subsequent crystal growth. Therefore, in the prior art semiconductor laser device, the current blocking layer 9, the GaAs layer 10, and the contact layer 11, which are grown on the etch stopping layer 6, have a density of crystalline defects, i.e., dislocations, as high as 1,000,000/cm2. In this case, laser light exuded from the upper cladding layer 5 is absorbed in the dislocations and the dislocations are increased, whereby the laser characteristics are degraded, resulting in a reduction in the reliability of the semiconductor laser device.

As described above, the stress in the crystalline structure of the laser is caused by that the lattice constant of the AlGaAs cladding layers 3, 5 and 7 is larger than the lattice constant of the GaAs substrate 1 and the GaAs contact layer 11. So, as a countermeasure against this problem, employing InGaP or AlInGaP having a lattice constant equivalent to the lattice constant of GaAs for the cladding layers is thought of. However, InGaP and AlInGaP have the following drawbacks. That is, InGaP has an energy band gap equivalent to a wavelength of about 650 nm, and this is smaller than the energy band gap equivalent to a wavelength of 615 nm that is required for preventing the overflow of charge carriers. Therefore, if InGaP is employed for the cladding layers, the overflow of charge carriers occurs, and the temperature characteristics of the laser are degraded. On the other hand, AlInGaP crystal easily takes oxygen when it is grown and, therefore, it is difficult to grow a high-quality AlInGaP crystal. In addition, it is not possible to increase the p type carrier concentration.

Furthermore, the crystalline defects in the current blocking layer 9 are caused by that the current blocking layer 9 is grown on the surface of the etch stopping layer 6 comprising a crystalline material that has a high Al composition and, therefore, is easily oxidized. As a countermeasure against the crystalline defects, a GaAs layer or an AlGaAs layer having a low Al composition is disposed under the etch stopping layer 6 having a high Al composition, and the etch stopping layer 6 is selectively removed with HCl. In this case, however, since the GaAs or Al GaAs layer under the etch stopping layer 6 has a small energy band gap and a large refractive index, this layer must be very thin to avoid degradation of laser characteristics due to absorption of generated laser light.

For example, a GaAs layer thinner than 20 A or an A1xGal~xAs (x = 0.2) layer thinner than 50 A is employed. However, when the layer under the etch stopping layer 6 is as thin as described above, it is difficult to selectively remove the etch stopping layer 6.

SUMMARY OF THE TNVNTTON It is an object of the present invention to provide a semiconductor laser device that prevents the degradation of laser characteristics and increases the reliability by reducing the stress in the crystalline structure of the laser and the crystalline defects.

Other objects and advantages of the invention will become apparent from the detailed description that follows.

The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.

According to a first aspect of the present invention, a semiconductor laser device comprises a first conductivity type GaAs substrate; a first conductivity type first lower cladding layer disposed on the GaAs substrate, latticematching with the GaAs substrate and having an energy band gap; a first conductivity type AlGaAs second lower cladding layer disposed on the first lower cladding layer and having an energy band gap larger than the energy band gap of the first lower cladding layer; an active layer disposed on the second lower cladding layer and having an energy band gap smaller than the energy band gap of the first lower cladding layer; a second conductivity type AlGaAs second upper cladding layer disposed on the active layer and having an energy band gap; a second conductivity type first upper cladding layer disposed on the second upper cladding layer, lattice-matching with the GaAs substrate, and having an energy band gap larger than the energy band gap of the active layer and smaller than the energy band gap of the second upper cladding layer; second conductivity type GaAs contact layer disposed on the first upper cladding layer; and first and second electrodes respectively disposed on the substrate and the contact layer. In this structure, unwanted overflow of charge carriers is prevented by the second lower cladding layer and the second upper cladding layer, and the thicknesses of these cladding layers for the prevention of the overflow of charge carriers can be reduced. Further, since the first lower cladding layer lattice-matches with the GaAs substrate and the second upper cladding layer lattice-matches with the GaAs substrate and the GaAs contact layer, a stress applied to the active layer is reduced, whereby degradation of laser characteristics is avoided.

According to a second aspect of the present invention, a semiconductor laser device comprises a first conductivity type semiconductor substrate; a first conductivity type lower cladding layer disposed on the semiconductor substrate and having an energy band gap; an active layer disposed on the lower cladding layer and having an energy band gap smaller than the energy band gap of the lower cladding layer; a second conductivity type first upper cladding layer disposed on the active layer and having an energy band gap larger than the energy band gap of the active layer; a second conductivity type InGaP first etch stopping layer disposed on the first upper cladding layer and having an energy band gap larger than the energy band gap of the active layer; a ridge structure comprising a second conductivity type second etch stopping layer disposed on a central portion of the first etch stopping layer and a second conductivity type second upper cladding layer disposed on the second etch stopping layer, the second etch stopping layer comprising a material having an energy band gap larger than the energy band gap of the active layer, the material being resistant to an etchant that etches the second upper cladding layer and being selectively etched by an etchant that does not etch the first etch stopping layer; first conductivity type current blocking layers disposed on the first etch stopping layer, contacting both sides of the ridge structure; and first and second electrodes respectively disposed on the substrate and the contact layer. In this structure, since the current blocking layer is produced on the InGaP etch stopping layer that is not easily oxidized, the crystalline defect density in the current blocking layer is significantly reduced. Further, since the energy band gap of the InGaP etch stopping layer is larger than the energy band gap of the active layer, the InGaP etch stopping layer does not absorb laser light, so that the laser characteristics are not degraded.

According to a third aspect of the present invention, a method of fabricating a semiconductor laser device comprises successively epitaxially growing on a first conductivity type GaAs substrate a first conductivity type InGaP first lower cladding layer having an energy band gap, a first conductivity type AlGaAs second lower cladding layer having an energy band gap larger than the energy band gap of the first lower cladding layer, an active layer having an energy band gap smaller than the energy band gap of the second lower cladding layer, a second conductivity type AlGaAs second upper cladding layer having an energy band gap larger than the energy band gap of the active layer, and a second conductivity type InGaP first upper cladding layer having an energy band gap smaller than the energy band gap of the second upper cladding layer; and epitaxially growing a GaAs contact layer on the first upper cladding layer. In a semiconductor laser device fabricated by this method, unwanted overflow of charge carriers is prevented by the AlGaAs second lower cladding layer and the AlGaAs second upper cladding layer, and the thicknesses of these cladding layers for the prevention of the overflow of charge carriers can be reduced. Further, since the InGaP first lower cladding layer lattice-matches with the GaAs substrate and the InGaP second upper cladding layer lattice-matches with the GaAs substrate and the GaAs contact layer, a stress applied to the active layer is reduced, whereby degradation of laser characteristics is avoided.

According to a fourth aspect of the present invention, a method of fabricating a semiconductor laser device comprises successively epitaxially growing on a semiconductor substrate a first conductivity type lower cladding layer, an active layer, a second conductivity type first upper cladding layer, a second conductivity type InGaP first etch stopping layer, a second etch stopping layer, and a second conductivity type second upper cladding layer; removing portions of the second upper cladding layer with a first etchant to form a ridge structure on a prescribed region of the second etch stopping layer; removing portions of the second etch stopping layer in regions other than the region where the ridge structure is present, with a second etchant, thereby exposing the first etch stopping layer; and epitaxially growing a first conductivity type current blocking layer on the first etch stopping layer that is exposed by the etching of the second etch stopping layer, contacting both sides of the second etch stopping layer and the ridge structure. In this method, since the current blocking layer is grown on the InGaP etch stopping layer that is not easily oxidized, the crystalline defect density in the current blocking layer is significantly reduced.

BRIEF flFSCRTPTION OF THE DRAWINGS Figure 1 is a ^-^s-s-sectional view illustrating a semiconductor laser device in accordance with a first embodiment of the present invention.

Figure 2 is a cross-sectional view illustrating a semiconductor laser device in accordance with a second embodiment of the present invention.

Figure 3 is a cross-sectional view illustrating a semiconductor laser device in accordance with a third embodiment of the present invention.

Figures 4(a)-4(e) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser device shown in figure 3.

Figure 5 is a cross-sectional view illustrating a semiconductor laser device in accordance with a fourth embodiment of the present invention.

Figures 6(a)-6(e) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser device shown in figure 5.

Figure 7 is a cross-sectional view illustrating a semiconductor laser device according to the prior art.

Figures 8(a)-8(d) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser device shown in figure 7.

DETAILED DESCRTPTION OF THE PREFERRED EMRODIMENTS [Embodiment 1] Figure 1 is a cross-sectional view illustrating a semiconductor laser device in accordance with a first embodiment of the present invention.

The semiconductor laser device shown in figure 1 includes a first conductivity type GaAs substrate 101 having opposed front and rear surfaces. A first conductivity type first lower cladding layer 112 having an energy band gap and lattice-matching with the GaAs substrate 101 is disposed on the front surface of the GaAs substrate 101. A first conductivity type AlGaAs second lower cladding layer 113 having an energy band gap larger than the energy band gap of the first lower cladding layer 112 is disposed on the first lower cladding layer 112. An active layer 104 having an energy band gap smaller than the energy band gap of the first lower cladding layer 112 is disposed on the second lower cladding layer 113. A second conductivity type AlGaAs second upper cladding layer 114 having an energy band gap is disposed on the active layer 104. A second conductivity type first upper cladding layer 117 having an energy band gap larger than the energy band gap of the active layer 104 and smaller than the energy band gap of the second upper cladding layer 114 and lattice-matching with the GaAs substrate 101 is disposed on a central portion of the second upper cladding layer 114. A second conductivity type cap layer 108 is disposed on the first upper cladding layer 117.

The first upper cladding layer 117 and the cap layer 108 form a ridge structure. First conductivity type current blocking layers 109 are disposed on the second upper cladding layer 114 at both sides of the ridge structure.

Second conductivity type GaAs layers 110 are disposed on the current blocking layers 109. A second conductivity type GaAs contact layer 111 is disposed on the cap layer 108 and on the GaAs layers 110.

In this semiconductor laser device, each of the upper cladding layer and the lower cladding layer has a doublelayer structure. More specifically, the first lower cladding layer 112 that lattice-matches with the GaAs substrate 101 and has an energy band gap larger than the energy band gap of the active layer 104 is disposed on the GaAs substrate 101, and the first upper cladding layer 117 that lattice-matches with the GaAs substrate 101 and the GaAs contact layer 111 and has an energy band gap equivalent to the energy band gap of the first lower cladding layer 112 is disposed under the GaAs contact layer 111 with the cap layer 108 between them. Thereby, the stress within the crystalline structure of the laser is reduced. Furthermore, the AlGaAs second lower cladding layer 113 having an energy band gap larger than the energy band gap of the first lower cladding layer 112 is disposed on the first lower cladding layer 112, and the AlGaAs second upper cladding layer 114 having an energy band gap larger than the energy band gap of the first upper cladding layer 117 is disposed under the first upper cladding layer 117. Although to decrease the lattice constant is inconsistent with to increase the energy band gap in AlGaAs, when the above-described double-layer cladding structure is employed, it is possible to give priority to the energy band gap over the lattice constant for the AlGaAs second lower cladding layer 113 and the AlGaAs second upper cladding layer 114. Therefore, unwanted overflow of charge carriers can be prevented by appropriately selecting, i.e., increasing, the energy band gaps of the second lower cladding layer 113 and the second upper cladding layer 114.

Although the double-layer cladding structure must have a sufficient thickness to prevent laser light generated in the active layer 104 from reaching into the GaAs substrate 1, the cap layer 108, and the contact layer 111, the abovedescribed overflow preventing effect is achieved even when the thicknesses of the second lower cladding layer 113 and the second upper cladding layer 114 are reduced. On the other hand, the amount of the stress produced in the crystalline structure of the laser due to the large lattice constants of the second lower cladding layer 113 and the second upper cladding layer 114 depends on the thicknesses of these cladding layers 113 and 114. So, when the thicknesses of the second lower cladding layer 113 and the second upper cladding layer 114 are reduced as thin as possible and the thicknesses of the first lower cladding layer 112 and the first upper cladding layer 117 are increased, both the prevention of the overflow of charge carriers and the reduction in the stress in the crystalline structure are realized at the same time, which was difficult in the prior art laser device. Therefore, in the semiconductor laser device according to the first embodiment, the stress applied to the active layer 104 is reduced, so that no crystalline defects are produced during the high-power output operation of the laser. As a result, degradation of laser characteristics due to crystalline defects is avoided, whereby the reliability of the device is improved.

[Embodiment 2] Figure 2 is a cross-sectional view illustrating a semiconductor laser device in accordance with a second embodiment of the present invention. In the figure, the same reference numerals as those in figure 1 designate the same or corresponding parts.

The semiconductor laser device shown in figure 2 includes a first conductivity type GaAs substrate 101 having opposed front and rear surfaces. A first conductivity type lower cladding layer 103 having an energy band gap is disposed on the front surface of the GaAs substrate 101. An active layer 104 having an energy band gap smaller than the energy band gap of the lower cladding layer 103 is disposed on the lower cladding layer 103. A second conductivity type first upper cladding layer 105 having an energy band gap larger than the energy band gap of the active layer 104 is disposed on the active layer 104. A second conductivity type InGaP first etch stopping layer 115 having an energy band gap larger than the energy band gap of the active layer 104 is disposed on the first upper cladding layer 105. A second conductivity type second etch stopping layer 116 is disposed on a portion of the first etch stopping layer 115.

A second conductivity type second upper cladding layer 107 is disposed on the second etch stopping layer 116. A second conductivity type cap layer 108 is disposed on the second upper cladding layer 107. The second etch stopping layer 116, the second upper cladding layer 107, and the cap layer 108 form a ridge structure. First conductivity type current blocking layers 109 are disposed on the first etch stopping layer 115 at both sides of the ridge structure. Second conductivity type GaAs layers 110 are disposed on the current blocking layers 109. A second conductivity type GaAs contact layer 111 is disposed on the cap layer 108 and on the GaAs layers 110. In this structure, the second etch stopping layer 116 comprises a material having an energy band gap larger than the energy band gap of the active layer 104 and being resistant to an etchant that etches the second upper cladding layer 107. Further, the material can be selectively etched with an etchant that does not etch the first etch stopping layer 115.

In the semiconductor laser device according to this second embodiment, the current blocking layer 109 is disposed on the first etch stopping layer 115 comprising InGaP. Therefore, when the current blocking layer 109 is grown by MOCVD, the surface of the substrate where this layer is grown is made of InGaP that is not easily oxidized, so that no surface oxidation occurs. Therefore, the crystal growth of the current blocking layer 109 is significantly facilitated, and the crystalline defect density in the current blocking layer 109 is significantly reduced. In addition, since the InGaP first etch stopping layer 115 has an energy band gap larger than the energy band gap equivalent to the wavelength of laser light produced in the active layer 104 and has a sufficiently small refractive index, it does not absorb the laser light to degrade the laser characteristics. As a result, the laser device according to the second embodiment of the invention avoids the problem of the prior art laser device, that is, degradation of laser characteristics due to absorption of laser light exuded from the upper cladding layer in dislocations and increase of the dislocations, resulting in an increase in the reliability.

Furthermore, the second etch stopping layer 116 is disposed on the first etch stopping layer 115, and the second etch stopping layer 116 comprises a material which is not etched by an etchant that etches the second upper cladding layer 107 but is etched by an etchant that does not etch the first etch stopping layer 115. Therefore, when the ridge structure is formed, the second upper cladding layer 107 is etched selectively with respect to the second etch stopping layer 116, whereby the controllability of the ridge formation is improved. Thereafter, the second etch stopping layer 116 is etched selectively with respect to the InGaP first etch stopping layer 115 to expose the InGaP layer 115 at the surface of the subst semiconductor laser device in accordance with a third embodiment of the present invention. In the figure, reference numeral 1 designates an n type GaAs substrate having opposed front and rear surfaces. An n type GaAs buffer layer 2 having a thickness of 1 urn is disposed on the front surface of the n type GaAs substrate 1. An n type InGaP first lower cladding layer 12 having a thickness of 1.5 urn is disposed on the buffer layer 2. An n type AlxGa1 xAs (Al composition x = 0.48) second lower cladding layer 13 having a thickness of 0.3 urn is disposed on the first lower cladding layer 12. An active layer 4 is disposed on the second lower cladding layer 13. The active layer 4 has a quantum well structure in which AlxGal~xAs (x = 0.1) well layers and AlxGal~xAs (x = 0.35) barrier layers are alternatingly laminated. A p type AlxGal~xAs (x = 0.48) second upper cladding layer 14 having a thickness of 0.3 urn is disposed on the active layer 4. A p type InGaP first etch stopping layer 15 having a thickness of 0.02 urn is disposed on the second upper cladding layer 14. A p type AlxGal~xAs (x = 0.48) second etch stopping layer 16 having a thickness of 0.02 urn is disposed on a portion of the first etch stopping layer 15. A p type InGaP first upper cladding layer 17 having a thickness of 1.5 urn is disposed on the second etch stopping layer 16. A p type GaAs cap layer 8 having a thickness of 0.1 urn is disposed on the -first upper cladding layer 17. The second etch stopping layer 16, the first upper cladding layer 17, and the cap layer 8 form a ridge structure. N type GaAs current blocking layers 9 having a thickness of 1 urn and p type GaAs layers 10 having a thickness of 0.6 urn are successively disposed on regions of the first etch stopping layer 15 where the ridge structure is absent, whereby the ridge structure is buried in these layers. A p type GaAs contact layer 11 having a thickness of 2 pm is disposed on the p type GaAs layers 10 and on the cap layer 8. A p side electrode 21 comprising Cr/Au is disposed on the contact layer 11. An n side electrode 20 comprising AuGe/Ni/Au is disposed on the rear surface of the substrate 1.

The composition of InGaP employed for the first lower cladding layer 12 and the first upper cladding layer 17 is desired to be InxGal~xP (x = 0.499 + 0.0065). When the composition of InGaP is within this range, the lattice mismatching degree of InGaP to GaAs that is employed for the substrate 1 and the contact layer 11 is ha/aO < Il x where Aa is the difference in lattice constants between the substrate 1 (contact layer 11) and the first lower cladding layer 12 (first upper cladding layer 17) in the direction perpendicular to the surface of the substrate 1 (contact layer 11), and a0 is the lattice constant of the substrate 1 (contact layer 11). Since this lattice mismatching degree is negligible, it can be said that the first lower cladding layer 12 (first upper cladding layer 17) substantially lattice-matches with the substrate 1 (contact layer 11).

In this third embodiment of the invention, the second upper cladding layer 14 and the second lower cladding layer 13 each comprising AlxGal~xAs (x = 0.48), having a thickness of 0.3 urn, and having an energy band gap that is larger than the energy band gap of the active layer 4 and sufficient to prevent overflow of injected charge carriers are disposed on the upper and lower surfaces of the active layer 4, respectively. Further, the first upper cladding layer 17 and the first lower cladding layer 12 each comprising InGaP, having a thickness of 1.5 pm, and having a lattice constant equivalent to the lattice constant of the GaAs contact layer 11 and the GaAs substrate 1 are disposed sandwiching the second upper cladding layer 14 and the second lower cladding 13. In this structure, the first upper cladding layer 17 and the first lower cladding layer 12 lattice-match with the GaAs contact layer 11 and the GaAs substrate 1, respectively, and the second lower cladding layer 13 and the second upper cladding layer 14, each comprising AlGaAs having a large lattice constant, are thinner than the thickness (1.5 pm) of the AlGaAs cladding layer of the prior art device. Therefore, the stress in the crystalline structure of the laser is reduced.

On the other hand, although the energy band gaps of the InGaP first cladding layer 12 and the InGaP first upper cladding layer 17 are not sufficient to prevent the overflow of injected charge carriers, since the AlGaAs second lower cladding layer 13 and the AlGaAs second upper cladding layer 14 disposed on the both sides of the active layer 4 have large energy band gaps, the overflow of injected charge carriers is prevented. A thickness of 0.1 pm is enough for the AlGaAs second lower cladding layer 13 and the AlGaAs second upper cladding layer 14 to prevent the overflow of injected charge carriers. Therefore, it is desired that the thicknesses of the AlGaAs second lower cladding layer 13 and the AlGaAs second upper cladding layer 14 are within a range from 0.1 pm to 0.5 urn, considering the stress produced in the crystalline structure. As a result, this third embodiment of the invention solves the problems of the prior art laser device, that is, the stress in the crystalline structure of the laser due to the AlGaAs cladding layers 3 and 7 and the overflow of charge carriers due to the InGaP layer.

A description is now given of the fabricating process of the semiconductor laser device according to this third embodiment of the invention. Figures 4(a)-4(e) are crosssectional views illustrating process steps in a method of fabricating the semiconductor laser device. In these figures, the same reference numerals as those in figure 3 designate the same or corresponding parts. Reference numeral 18 designates an SiO2 film, and reference numeral 19 designates a resist film.

Initially, there are successively grown on the n type GaAs substrate 1 by MOCVD, the n type GaAs buffer layer 2, the n type InGaP first lower cladding layer 12, the n type AlGaAs second lower cladding layer 13, the quantum well active layer 4, the p type AlGaAs second upper cladding layer 14, the p type InGaP first etch stopping layer 15, the p type AlGaAs second etch stopping layer 16, the p type InGaP first upper cladding layer 17, and the p type GaAs cap layer 8 (figure 4(a)).

Thereafter, an SiO2 film 18 is deposited over the p type GaAs cap layer 8 by sputtering, and a stripe-shaped resist film 19 is formed on the SiO2 film 18. Using the resist film 19 as a mask, the SiO2 film 18 is patterned.

Thereafter, using the resist film 19 and the SiO2 film 18 as masks, the p type GaAs cap layer 8 is etched with a mixture of NH4OH and H202 and, subsequently, the p type InGaP first upper cladding layer 17 is etched with a mixture of HCl and H20, thereby forming a ridge structure (figure 4(b)). Since the etching of the InGaP first upper cladding layer 17 with the mixture of HC1 and H20 stops at the p type AlGaAs second etch stopping layer 16, very high reproducibility is obtained.

Next, using the resist film 19 and the SiO2 film 18 as masks, portions of the p type AlGaAs second etch stopping layer 16 other than a portion where the ridge structure is present are selectively removed with HF to expose the first etch stopping layer 15 (figure 4(c)). After removal of the resist film 19, the n type GaAs current blocking layer 9 and the p type GaAs layer 10 are successively grown on the first etch stopping layer 15, contacting both sides of the ridge structure, by MOCVD (figure 4(d)). When the temperature is risen before the MOCVD growth, a PH3 ambient is produced in the MOCVD apparatus. Since the first etch stopping layer 15 is exposed at the surface when the growth is carried out and this layer 15 does not comprise AlGaAs that is easily oxidized but comprises InGaP that does not contain Al, the surface of the first etch stopping layer 15 is not oxidized, so that a crystal growth with less crystalline defect is realized.

After removal of the SiO2 mask 18 with HF, the p type GaAs contact layer 11 is grown on the p type GaAs cap layer 8 and on the p type GaAs layers 10 by MOCVD. Finally, the n side electrode 20 is produced on the rear surface of the GaAs substrate 1 and the p side electrode 21 is produced on the contact layer 11 to complete the semiconductor laser device (figure 4(e)).

In this third embodiment of the invention, as described above, the p type AlxGal~xAs (x = 0.48) second etch stopping layer 16 is present beneath the InGaP first upper cladding layer 17, and the p type InGaP first etch stopping layer 15 is present beneath the second etch stopping layer 16.

Therefore, when a mixture of HC1 and H202 is employed for the etching to produce the ridge structure, the etching stops at the AlGaAs second etch stopping layer 16 and only the InGaP first upper cladding layer 17 is selectively removed, whereby the ridge structure is produced with high controllability. After the formation of the ridge, the p type AlGaAs second etch stopping layer 16 is selectively etched with HF without etching the InGaP first upper cladding layer 17 at the side surfaces of the ridge, thereby exposing the p type InGaP first etch stopping layer 15 at the surface of the substrate. It is desired that the Al composition x of the p type AlxGal~xAs second etch stopping layer 16 is within a range from 0.3 to 0.5 because the second etch stopping layer 16 is etched with HF when the Al composition x is smaller than 0.3 and with the mixture of HC1 and H202 when the Al composition x is larger than 0.5.

Further, since the p type InGaP first etch-stopping layer 15 and the p type AlGaAs second etch stopping layer 16 have energy band gaps larger than the energy band gap that is equivalent to the wavelength of laser light produced in the active layer 4, and sufficiently small refractive indices, it is not necessary to make these layers 15 and 16 very thin. Therefore, there is no difficultly in removing the upper etch stopping layer alone without removing the lower etch stopping layer.

Furthermore, when the n type GaAs current blocking layer 9 is grown, since exposed at the surface of the substrate for the growth is not AlGaAs that is easily oxidized but InGaP that is hardly oxidized, the crystal growth of the current blocking layer 9 is significantly facilitated, and the crystalline defect density, i.e., the dislocation density, in the current blocking layer 9 is 2 reduced from 1,000,000/cm2 to 500/cm2.

In the structure before the crystal growth of the n type GaAs current blocking layer 9, InGaP is exposed at the surface of the substrate and the side surfaces of the ridge structure. Therefore, a PH3 ambient is produced in the MOCVD apparatus in the temperature rising process before the crystal growth, thereby preventing escape of P atoms from the InGaP crystalline structure and formation of a degenerate layer at the growth interface.

A description is given of the operation of the semiconductor laser device according to the third embodiment of the invention.

When a voltage is applied across the n side electrode 20 and the p side electrode 21, electrons and holes are injected into the active layer 4 from the electrodes 20 and 21, respectively, and recombine to produce laser light.

Since the stress in the crystalline structure of the semiconductor laser is reduced as compared to the prior art device, degradation in the laser characteristics due to dislocations in the crystalline structure is prevented even in the high-power output operation of the laser. In addition, since the crystalline defect density in the current blocking layer 9, the p type GaAs layer 10, and the contact layer 11 are significantly reduced as compared with the prior art laser device, degradation in the laser characteristics due to absorption of laser light exuded from the active layer 4 by the crystalline defects is prevented.

As a result, a highly-reliable AlGaAs semiconductor laser device is realized.

tEmbodiment 4] Figure 5 is a cross-sectional view illustrating a semiconductor laser device in accordance with a fourth embodiment of the present invention. In the figure, the same reference numerals as those shown in figure 3 designate the same or corresponding parts. Reference numeral 3 designates an n type AlxGal~xAs (x = 0.48) lower cladding layer having a thickness of 1.5, reference numeral 5 designates a p type AlxGa1xAS (x = 0.48) first upper cladding layer 5 having a thickness of 0.3 pm, reference numeral 6 designates a p type AlxGal~xAs (x = 0.7) second etch stopping layer having a thickness of 0.02 pm, and numeral 7 designates a p type AlxGal~xAs (x = 0.48) second upper cladding layer having a thickness of 1.2 pm.

Figures 6(a)-6(e) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser shown in figure 5.

A description is given of the fabricating process.

Initially, there are successively grown on the n type GaAs substrate 1 the buffer layer 2, the lower cladding layer 3, the active layer 4, the first upper cladding layer 5, the first etch-stopping layer 15, the second etch-stopping layer 6, the second upper cladding layer 7, and the cap layer 8 (figure 6(a)). These layers are grown by crystal growth using MOCVD method. Thereafter, an Si02 film 18 is deposited over the p type GaAs cap layer 8 by sputtering, and a stripe-shaped resist film 19 is formed on the SiO2 film 18. Using the resist film 19 as a mask, the SiO2 film 18 is patterned. Thereafter, using the resist film 19 and the SiO2 film 18 as masks, the p type GaAs cap layer 8 is etched with a mixture of NH40H and H202 and, subsequently, the second upper cladding layer 7 is etched with a mixture of tartaric acid and hydrogen peroxide, thereby producing a ridge structure (figure 6(b)). Since the etching of the second upper cladding layer 7 stops at the p type AlxGal~xAs (x = 0.7) second etching stopper layer 6, the ridge structure is produced with high reproducibility. Then, the p type AlGaAs second etch stopping layer 6 is selectively removed with a mixture of HF and H20 to expose the InGaP first etch stopping layer 15 at the surface of the substrate (figure 6(c)). After removal of the resist film 19, the n type GaAs current blocking layer 9 and the p type GaAs layer 10 are successively grown on the p type InGaP first etch stopping layer 15, contacting the both sides of the ridge structure, using MOCVD (figure 6(d)). When the temperature is risen before the MOCVD growth, a PH3 ambient is produced in the MOCVD apparatus.

After removal of the SiO2 mask 18 with HF, the p type GaAs contact layer 11 is grown on the p type GaAs cap layer 8 and on the p type GaAs layer 10 by MOCVD. To complete the laser device, the n side electrode 20 is produced on the rear surface of the substrate 1 and the p side electrode 21 is produced on the contact layer 11 (figure 6(e)).

In this fourth embodiment of the invention, the p type InGaP first etch stopping layer is disposed beneath the p type AlxGal~xAs (x = 0.7) second etch stopping layer 6 that is easily oxidized and makes crystal growth thereon difficult. Therefore, when a mixture of tartaric acid and hydrogen peroxide is employed for the etching to produce the ridge structure, the etching stops at the p type AlGaAs second etch stopping layer 6 and only the AlGaAs second upper cladding layer 7 is selectively removed, whereby the ridge structure is produced with high controllability.

After the formation of the ridge, the p type AlGaAs second etch stopping layer 6 is selectively etched with a mixture of HF and H20 without etching the AlxGal~xAs (x = 0.48) second upper cladding layer 7 at the side surfaces of the ridge, thereby exposing the p type InGaP first etch stopping layer 15 at the surface of the substrate. It is desired that the Al composition x of the p type AlxGal~xAs second etch stopping layer 6 is higher than 0.6 because the AlGaAs second etch stopping layer 6 is easily etched with a mixture of tartaric acid and hydrogen peroxide when the Al composition x is smaller than 0.6.

In this fourth embodiment of the invention, since the InGaP first etch stopping layer 15 is employed, the crystalline defect density in the n type GaAs current blocking layer 9 is reduced as in the above-described third embodiment, whereby a highly-reliable semiconductor laser device is realized.

In this fourth embodiment of the invention, however, in the structure before the growth of the n type GaAs current blocking layer 9, the p type InGaP first etch stopping layer 15 is exposed at the surface of the substrate, and the p type AlGaAs upper cladding layer 7 and the p type GaAs cap layer 8 are exposed at the side surfaces of the ridge structure. That is, the surface of the structure comprises two kinds of V elements, i.e., P and As. In this case, it is difficult to prevent evaporation of the V elements during the growth temperature rising process. More specifically, when a PH3 ambient is produced in the MOCVD apparatus in the temperature rising process to prevent evaporation of P atoms from the p type InGaP first etch stopping layer 15, As atoms are unfavorably evaporated from the p type AlGaAs upper cladding layer 7 and the p type GaAs cap layer 8. When an AsH3 ambient is produced in the MOCVD apparatus, degenerate layers are formed at the surfaces of the p type AlGaAs upper cladding layer 7, the p type GaAs cap layer 8, and the p type InGaP first etch stopping layer 15.

However, influences of the evaporation of As atoms from the upper cladding layer 7 and the cap layer 8 on the reliability of the laser device are negligible for the following reasons. That is, the n type GaAs current blocking layer 9 can be grown at a relatively low temperature, about 600"C, and As atoms in the p type AlGaAs upper cladding layer 7 having such high Al composition (0.48) are relatively stable and hardly evaporated at the low temperature. In addition, since the p type GaAs cap layer 8 is away from the active layer 4, the evaporation of As atoms from the p type GaAs cap layer 8 does not adversely affect the laser characteristics. Therefore, in order to produce a high-quality n type GaAs current blocking layer with reduced defect density that ensures a highly-reliable semiconductor laser device, it is important to prevent the evaporation of P atoms from the p type InGaP first etch stopping layer 15 by making the PH3 ambient in the MOCVD apparatus in the temperature rising process before the growth of the current blocking layer.

Claims (6)

1. A semiconductor laser device comprising: a first conductivity type GaAs substrate; a first conductivity type first lower cladding layer disposed on the GaAs substrate, lattice-matching with the GaAs substrate, and having an energy band gap; a first conductivity type AlGaAs second lower cladding layer disposed on the first lower cladding layer and having an energy band gap larger than the energy band gap of the first lower cladding layer; an active layer disposed on the second lower cladding layer and having an energy band gap smaller than the energy band gap of the first lower cladding layer; a second conductivity type AlGaAs second upper cladding layer disposed on the active layer and having an energy band gap; a second conductivity type first upper cladding layer disposed on the second upper cladding layer, latticematching with the GaAs substrate, and having an energy band gap larger than the energy band gap of the active layer and smaller than the energy band gap of the second upper cladding layer; second conductivity type GaAs contact layer disposed on the first upper cladding layer; and first and second electrodes respectively disposed on the substrate and the contact layer; wherein the first lower cladding layer comprises first conductivity type InGaP, and the first upper cladding layer comprises second conductivity type InGaP.

2. The semiconductor laser of claim 1 wherein the second upper cladding layer and the second lower cladding layer have thicknesses within a range from 0.1 pm to 0.5 pm.

3. A method of fabricating a semiconductor laser device comprising: successively epitaxially growing on a first conductivity type GaAs substrate a first conductivity type InGaP first lower cladding layer having an energy band gap, a first conductivity type AlGaAs second lower cladding layer having an energy band gap larger than the energy band gap of the first lower cladding layer, an active layer having an energy band gap smaller than the energy band gap of the second lower cladding layer, a second conductivity type AlGaAs second upper cladding layer having an energy band gap larger than the energy band gap of the active layer, and a second conductivity type InGaP first upper cladding layer having an energy band gap smaller than the energy band gap of the second upper cladding layer; and epitaxially growing a GaAs contact layer on the first upper cladding layer.

4. The method of claim 3 including: after the epitaxial growth of the layers from the first lower cladding layer to the first upper cladding layer, etching and removing portions of the first upper cladding layer with an etchant to form a ridge structure.

5. The method of claim 4 wherein the etchant is a mixture of HC1 and H20.

6. A semiconductor laser device substantially as hereinbefore described with reference to Figure 1; or Figure 2, or Figures 3 and 4; or Figures 5 and 6; or Figures 7 and 8 of the accompanying drawings.

7 A method of manufacturing a semiconductor laser device substantially as hereinbefore described with reference to Figure 1; or Figure 2, or Figures 3 and 4; or Figures 5 and 6; or Figures 7 and 8 of the accompanying drawings.